Determination of metal-humic complexes, free metal ions and total concentrations in natural waters

A comparison of two systems for the quantitation of metal-humic complexes a nd free metal ions, consisting of the separation by coupled ion exchange co lumns followed by detection by inductively coupled plasma mass spectrometry or cold vapour atomic fluorescence spectrometry, is presented. The systems evaluated comprised the serially coupled anion and cation exchangers, Seph adex A-25/Chelex 100 and Dowex 1X8/Chelamine Metalfix. Separation and preco ncentration of the species studied were accomplished with both systems, elu tion being carried out using 2 M HNO3. Total concentrations, metal-humic co mplex fractions and free metal ion fractions of Al, Ba, Cd, Co, Cu, Fe, Hg, Mn, Pb, Sr, U and Zn in nine natural waters were determined. Statistical e valuation of the data from the two cation exchange materials, including res ults for additional elements, showed better precision (for Al, Ba. Cr, Cu a nd Mo) and higher recoveries (Al, Ba, Cd, Fe, Sr and Zn) for Chelex 100 tha n Chelamine Metalfix for free metal ions. On the other hand, Chelamine Meta lfix recovered a significantly greater amount of Ni. The amounts of metal-h umic complexes were compared with modelled distributions of these species, and one advantage of the preferred Sephadex A-25/Chelex 100 system is that the elements studied are all correctly classified with respect to their bin ding strengths to humic substances, which is not the case with the Dowex 1X 8/Chelamine Metalfix pair. With the preferred system, metal-humic complexes can be reliably determined, as indicated by the results of equilibrium spe ciation modelling. However, comparison with the total concentrations showed statistically significant, non-quantitative recoveries of Al, Cu, Hg, Mn. U and Zn from some samples. Thus a combination of speciation and total conc entration measurements is required to obtain a complete representation of t he distribution of trace elements in natural waters.